1. Field of the Invention
The present invention relates to a method for manufacturing a color filter array, particularly one used in flat panel color displays such as liquid crystal display devices, and other optoelectric devices.
2. Description of the Prior Art
Liquid crystal displays (LCD) and other flat panel display devices (FPD) are known for monochrome digital display in, for example, electronic calculators, clocks, household appliances, audio equipment, etc. There has been a need to introduce a color display capability into such monochrome display devices. The need arises particularly for applications such as peripheral terminals using various kinds of equipment involving phototube display, mounted electronic display, or TV-image display.
Although previous attempts have been made to introduce a color array into these devices, none of the color arrays for flat panel display devices so far proposed have been acceptable in meeting all the users needs. Materials and methods used to fabricate, for example, a red, green and blue (RGB) color filter array, for full color displays, in the past have included gelatin dyeing, dyed polyimide, pigmented polyimide and pigmented acrylates, applied by photolithographic printing and electrodeposition. However, these methods have low yields and throughput in high definition photolithographic application. Color filter materials add 32.2% to the cost of LCD production. Added to the expense of such materials is the fact that their shelf stability is not good.
In conventional gelatin dyeing a negative photolithographic imaging method is employed. The color display is made by photosensitizing a gelatin layer, exposing the layer to a pattern of light shining through a mask, developing the pattern, hardening the gelatin in the exposed areas, and washing to remove the unexposed gelatin. The pattern of gelatin thus produced is then colored by the first dye solution of the desired array. The pattern is then recoated with gelatin and the above steps are repeated for each additional color desired. This method contains many labor-intensive steps, requires careful alignment, separate protective coatings, and is time-consuming and unduly expensive. Further details of the gelatin dyeing process are exemplified in U.S. Pat. No. 4,081,227.
Another photolithographic method for making a color filter array is directed to dying polyimides as disclosed in U.S. Pat. No. 4,876,165. The method includes making the color filter array by positive-imaging photolithography. This technique deals with a positive photoresist patterning process where the exposed region, rather than the re-exposed region, is washed away leaving a pattern produced from raised lines rather than by negative recesses. The use of soluble dyes, dissolved in the polyimide, produces a homogeneous coating. This homogeneous coating enhances color uniformity. The polyimide matrix provides high thermal stability (to 230.degree. C.), rendering the final color filter compatible with indium tin oxide (ITO) processes. A coplanar pixel display may be produced by patterning one level and then overcoating polyimide for patterning of the next level.
A similar dyed polyimide to make a color filter array is disclosed in U.S. Pat. No. 5,176,971 by use of photolithographic processes disclosed in U.S. Pat. No. 5,147,844, U.S. Pat. No. 5,166,125, and U.S. Pat. No. 5,166,126 and by thermally transferring the dyes.
In Japanese Pat. No. 8493679 pigmented polyimide color filter materials are employed. In a number of Japanese patents, e.g. JP04330405A, JP 9191260, JP 831216, JP 60129739, and JP 92037987, pigmented dispersions are used as the color filter materials. In these methods photolithographic processing is the typical technique for making color filter arrays.
Off-set printing has also been used in panel display manufacturing to make color filter arrays for low end application, but are of low quality. Non-photolithographic methods for making color filter arrays are, for example, electrodeposition and vacuum deposition. These methods are not as common in manufacturing color filter arrays.
Another method for non-photolithographic color filter array manufacture in a liquid crystal display device is described in EPA No. 246,334. This method employs a porous membrane to contain dyes which are transferred by heat under reduced pressure using a metal mask. The dyes are transferred from a donor layer, to a receiver layer through an air gap formed therein between. This technique yields insufficiently sharp images. The drawback is characteristic of dye receiver materials.
In photolithographic processes, a minimum of eight (8) steps per color layer is required. Therefore, it requires at least thirty-two steps to make a full color filter array which would include three RGB color layers plus a black matrix. The multiplicity of steps in photolithographic processing results in low yield and throughput. For example if there is a 95% yield for each step, the final yield is only 19% for a 32-step process. In order to obtain an 80% final yield, the yield must be greater than 99.3% in each step. The best final yield to date for photolithographic display processes has been approximately 50%.
The high number of process steps resulting in low throughput and excessive equipment and extra labor are not the only problem. Also, the chemicals involved in this process may pose environmental problems, stemming from hazardous waste by-products.
In addition, color filter arrays used for flat panel devices are required to undergo severe heating during treatment and processing. For example, a transparent electrode layer, such as indium tin oxide (ITO), is usually vacuum sputtered onto the color filter array at temperatures as high as 300.degree. C., and over a period of one hour or more. This is followed by coating with a thin alignment layer. The surface of the alignment layer which contacts the liquid crystals may require rubbing or curing for several hours at an elevated temperature. Such treatment steps threaten the colored filter material of the array, especially those with a gelatin matrix.
The individual color regions or pixel regions created by the above methods range in size from 20 to 600 .mu.m square.
Laser Ablation has been discussed in the prior art although not in the manufacturing of color filter arrays. U.S. Pat. No. 5,061,341 discloses a method of laser marking plastic articles. The method comprises a series of contrasting color plastic coatings. The laser ablates the upper layer enabling a message to be seen clearly. U.S. Pat. No. 4,624,736 discloses a method for using a laser to etch and deposit material on a substrate. The method uses a laser and a reactive gas near the substrate to modify the substrate. U.S. Pat. No. 4,877,644, discloses a method for treating a resist covered metal substrate so as to allow for easier removal of portions of the resist from the substrate surface. U.S. Pat. No. 4,478,677, discusses a method to etch glass to facilitate the mounting of high density circuit chips. The laser is used to excite the glass which in turn activates a gas that etches the glass substrate. U.S. Pat. No. 4,925,523, reveals a method to use two lasers to improve ultraviolet laser ablation. Finally, U.S. Pat. No. 5,170,191, is a method using laser ablation to shape an optical surface, specifically like soft contact lenses.
Laser ablation has been researched on a variety of polymers. Excimer lasers have been used to pattern polyvinylidene difluoride complex lines as narrow as 20 .mu.m, see Gauthier, M. et all, "Excimer laser thin metallic film patterning on polyvinylidene difluoride". Photoetching of a number of polymeric materials including poly(methyl-methacrylate) (PMMA), poly vinylacetate (PVA), poly (.alpha.-methyl styrene) (PS), poly (tetrafluoroethylene) (PTFE), polyproylene (PP), nitrocellulose, copolymer styrene allyl alcohol (SAA) and polymer-monomer mixture PVA plus 25 wt % biphenyl carbonitrile has been measured, see Liu, Y. et al, "Photoetching of Polymers with Excimer Lasers". This document included measurements of photoetch rates, analysis of optical absorption coefficients, and etch depth per pulse. Rates of photoetching were found to be dependent upon the optical absorption coefficient of polymeric materials and upon chemical structures of the polymer itself. Apparently exposed materials are modified by the initial laser pulses and then are etched away by continued exposure to the radiation. The etch rate (etched material per pulse) at a given exposure condition was thought to follow Beer's law. The basis of the technique is the well known process of ablative photodecomposition which was first mentioned in the literature in 1982. When organic polymers are exposed to laser light with sufficient photon energy and photon flux, they can undergo a transition to the gas phases so suddenly that the thermal effect on the surrounding material is minimal. Both photochemical and photothermal mechanism are thought to be factors depending on the material and the process conditions, see Srinivasan, R. et al, "Ultraviolet laser ablation of polyimide films".